Method and device for controlling a multi electrode sweep

ABSTRACT

The invention relates to a method and a device for sweeping marine mines having a magnetic sensor by at least three electrodes (10, 11, 13) spaced apart and tractored by a vessel (12) and behind each other, the electrodes being provided with electric current from the vessel (12) for generating a magnetic field in the water surrounding the electrodes (10, 11, 13), each of the electrodes (10, 11, 13) separately being provided with electric current of individually adjustable strength. The invention is characterized by varying in time the current strength of the current fed to the electrodes between positive and negative limits with intermediate zero passages to separate the time for zero passage of the current to at least one of the electrodes (10, 11, 13) from the time for zero passage of the current to the rest of the electrodes (10, 11, 13).

The present invention relates to a method and a device for sweepingmarine mines having a magnetic sensor using at least three electrodeswhich are spaced apart, and are tractored by a vessel and behind eachother, said electrodes being provided with electric current from saidvessel for generating a magnetic field in the water surrounding saidelectrodes, each of said electrodes separately being provided withelectric current of an individually adjustable strength.

When sweeping marine mines having a magnetic sensor a magnetic field hasto be generated, said magnetic field being sufficiently strong andsufficiently similar to a magnetic field generated by a vessel to beregarded by the mine as a vessel target, thereby detonating the mine.For the protection of the vessel carrying out the mine sweeping it isdesirable to limit the magnetic field of such a strength to an areasafely distanced from the mine sweeping vessel, so as to prevent a minedetonated by said magnetic field from damaging said mine sweepingvessel.

A sweeping operation must fulfil two primary demands. A first demand isto make mines having a low sensitivity detonating even if they aredisplaced a large distance in the transverse direction of the track ofthe vessel and thereby being actuated by a comparatively weak magneticfield from the sweep. A second demand is that mines having a highsensitivity shall not be triggered within a certain security zonesurrounding the sweeping vessel. These claims are partially conflictingbecause a strong magnetic field required to achieve said first demandhampers the achievement of said second demand. Furthermore, thecharacteristics of the magnetic field generated by the sweep should besuch that it is identified by the mine as a magnetic field generated bya target vessel, even if the mine is provided with means for analyzingsurrounding magnetic fields.

The method of sweeping marine mines having a magnetic sensor by means ofan electrode sweeping arrangement comprises the following steps. Two ormore electrodes are placed in the water and tractored by one or severalvessels. The electrodes are supplied with electric current from saidtractoring vehicle, the current in the cables and through the watergenerating the desired magnetic field.

U.S. Pat. No. 2,937,611 discloses a system in sweeping marine mines bymeans of a plurality of vessels, each vessel being provided with a pairof electrodes. The system provides a pulsating magnetic field betweenthe electrodes. U.S. Pat. No. 2,397,209 relates to a system in minesweeping according to which a pulsating magnetic field is providedbetween two of the electrodes tractored by the vessel. A morecomplicated system in mine sweeping is disclosed in U.S. 3,946,696. Thesystem comprises two electrodes, a controlled current generator, and amagnetic field sensor. There is also included a control systemcontrolling the current through the electrodes dependant on the magneticfield in the vicinity of the mine sweeping vessel. By measuring themagnetic field adjacent to the mine sweeping vessel the desired safetyof the mine sweeping vessel can be obtained. SE,A, 8704069-7 relates toa method and a device in sweeping marine mines having a magnetic sensor.At least three electrodes are tractored spaced apart behind a vessel andbehind each other, and said electrodes separately are provided withelectric current of individually adjustable strength from said vesselfor generating a magnetic field in the water surrounding saidelectrodes.

Another simple constructive step to increase the protection of the minesweeping vessel without any impairing of the desired mine sweepingcapabilities is to extend the mine sweeping arrangement behind thevessel. However, practical problems in dealing with long cables limitthe length of the mine sweeping arrangements.

The magnetic field from a vessel moving normally and passing a minevaries in each position by time and can be regarded as combined bycomponents in three directions of the co-ordinates in space. In eachdirection the magnetic field varies in such a way that during somemoments the value of said magnetic field is zero. The moment of theseso-called zero passages do not coincide in said three directions, a factwhich is used by "intelligent" mines to avoid firing caused by a minesweeping arrangement as described above, said zero passages of saidarrangements coinciding in said three directions.

An object of the present invention is to accomplish a method forsweeping marine mines which are fired magnetically, said methodfulfilling the above described demands. The object is accomplished byproviding said generated magnetic field propagation characteristicshaving a sufficiently weak magnetic field in the vicinity of the minesweeping vessel and a magnetic field varying in time according to thesteps set out in claim 1.

The invention will be described in more detail by means of an embodimentby reference to the accompanying drawings, in which

FIG. 1 schematically shows a prior art three electrode sweep,

FIG. 2 is a graph showing the field propagation of the three electrodesweep according to FIG. 1,

FIG. 3 schematically shows a three electrode sweep according to thepresent invention,

FIG. 4 schematically shows an embodiment of the three electrode sweepaccording to the present invention,

FIG. 5 schematically shows an alternative embodiment of the threeelectrode sweep according to the present invention,

FIG. 6a and FIG. 6b are graphs showing how the current in two electrodesvaries in time, and

FIG. 6c-e are graphs showing how the magnetic field varies in a positionin the water in three directions in time.

As mentioned initially two partly contradictory demands have to beaccomplished when sweeping mines. The magnetic field must besufficiently strong to detonate mines in an area as large as possible.Using the mine sweep according to FIG. 1 a field propagation accordingto FIG. 2 can be accomplished. The mine sweep comprises a firstelectrode 10, a second electrode 11 and a third electrode 13. Thecurrent I₁ in said third electrode 13 and the current I₃ in the secondelectrode 11 are provided through a control and regulating unit 14 inturn being provided with electric current from a not shown power supplymeans. From FIG. 2 it is also clear how said electrodes are arranged online behind a tractoring vessel 12, said third electrode 13 beingarranged closest to said vessel, and said second electrode 11 being thelast electrode. The lines of flux indicate the magnetic field in termsof nT. The width of an area covered by a magnetic field 100 nT strong isjust above 400 m. Most mines will identify 100 nT as vessel target. Theflux density allowed in the vicinity of the mine sweeping vessel variesdepending on different factors, but should preferably be limited to 5nT.

A crucial factor of the field propagation characteristic of a threeelectrode sweeping arrangement is the relationship between the currentI₁ in the front electrode 13 and the current I₃ in the rear electrode11, the distances between electrodes 10, 11 and 13, and the way thesupplied current (and thereby also the magnetic field) varies in time.The distances between said electrodes are indicated in FIG. 2, and therelationship between I₁ and I₃ is 1, i.e. the strength and direction ofcurrent I₁ are equal to the strength and direction of current I₃. Eachof the electrodes in the electrode sweeping arrangement is suppliedseparately with current, and the current in each electrode is controlledindividually. To accomplish a magnetic sweep having the desiredpropagation characteristics the arrangement is first of all made with anappropriate consideration to the types of electrodes, the types ofcables and the distances between the electrodes. Starting with thesefundamentals the desired relationship between said current I₁ in saidfront electrode 13 and said current I₃ in said rear electrode 11 isdetermined. Said currents I₁, I₂ and I₃ are then adjusted to appropriatevalues so as to achieve the desired current relationship.

FIG. 3 shows an embodiment in principle of a device according to theinvention. A power supply means 15 provides through separate means eachelectrode in the sweeping arrangement with an individually controllablecurrent. To make possible a desired adjustment of the current supply tosaid electrodes with regard to time, and thereby also the magneticfield, in three space co-ordinate directions said power supply means 15is operatively connected to a control means 23 comprising a central unit21 and a memory unit 22 in which control data to said central unit foraccomplishing any desired sequencies of varying magnetic field isstored. In a simple embodiment said control means 23 comprises aconventional mechanical timer, and in a further developed embodimentsaid central unit 21 comprises a computer and said memory unit compriseselectronic memory chips and in some cases memories on magnetic media.The method according to the invention is described in more detail belowwith reference to FIG. 6.

FIG. 4 shows schematically an embodiment of the device according to theinvention. The power supply means 15 comprises a first generator 16,providing said rear electrode 11 with the current I₃, and a secondgenerator 17 providing said front electrode 13 with the current I₁. Saidgenerators also comprise a common terminator which is connected to saidcenter electrode 10 and through which said current I₂ is supplied.Control signals generated in said control means 23 are amplified in twodriver means 24, 25. If AC generators are used rectifiers are providedbetween said generators and said electrodes. Controlled rectifiers arepreferably used to make possible an adjustment of the current strength.The flow direction of currents can of course be reversed.

In the embodiment shown in FIG. 5 the power supply means comprising twocontrolled current rectifiers 18; 19 is connected to a generatorexisting on said vessel 12 through a transformer 20.

All electrodes and cables are of conventional type.

The method according to the invention will now be described in moredetail with reference to FIG. 6a-e. FIG. 6a is an example of how thecurrent I₁ in said front electrode 13 is varied in time by said controlmeans 23, and FIG. 6b shows a corresponding variation of the current I₃in said rear electrode 11. As is clear form FIG. 6a and FIG. 6b the zeropassage of I₃ is displaced T₀ s in relation to the zero passage of I₁.The period of the variation of the current I₁ is referred to as T, andT₀ should preferably be less than or equal to T/4. The variation of saidcurrent I₁ and I₃ results in a variation also of the magnetic field.FIG. 6c-e show the variation of the magnetic field in an arbitraryposition in the three space co-ordinate directions x, y and z. As aresult of the displacement T₀ also the zero passages of the magneticfield in said three directions are displaced, and it is ensured that thegenerated magnetic field to a high extent corresponds to the magneticfield of a vessel.

I claim:
 1. A method for sweeping marine mines of the type having amagnetic sensor, comprising the steps of:spacing at least threeelectrodes apart from one another; tractoring said at least threeelectrodes behind a vessel in longitudinally spaced relation to oneanother; supplying each of said at least three electrodes with electriccurrent from said vessel for generating a magnetic field in watersurrounding said at least three electrodes; separately supplying each ofsaid at least three electrodes with electric current of individuallyadjustable strength; varying said electric current in time betweenpositive and negative limits with intermediate zero passages to separatethe time for a zero passage of the electric current to at least onepreselected electrode of said at least three electrodes from the timefor a zero passage of the electric current to the other than said atleast one preselected electrode.
 2. The method according to claim 1,further comprising the step of supplying electric current to theelectrode closest to said vessel in offset phase relation to theelectric current to the electrode arranged most distant from saidvessel.
 3. The method according to claims 1 or 2, further comprising thestep of varying the strength of the electric current while maintaining apredetermined relationship between the electric current to the electrodeclosest to said vessel and the electric current to the electrodearranged most distant from said vessel.
 4. The method of claims 1 or 2,further comprising the step of setting the difference in time betweenzero passages of the strength of the current of the electrode closest tosaid vessel and the strength of the current to the electrode mostdistant from said vessel to a preselected time that is below one fourthof the time interval between two zero passages of one of the currents.5. The method of claims 1 or 2, further comprising the steps ofproviding a first electrode, a second electrode and a third electrode insequence behind said vessel substantially along a straight line, saidfirst electrode arranged closest to said vessel, and adjusting thecurrent of said first electrode and the current of said third electrodeto a predetermined relationship considering the size of said electrodesand the distance therebetween, and adjusting the current of a centerelectrode to a value adequate to provide predetermined propagationcharacteristics of the magnetic field generated by said electrodes.
 6. Avessel-towed device for sweeping marine mines of the type having amagnetic sensor, comprising:at least three electrodes towed by saidvessel in spaced, longitudinal alignment with one another and saidvessel; a power supply means arranged on said vessel for supplyingcurrent of individually adjustable strength to said electrodes; saidpower supply means being connected to control means for time coordinatedcontrol of the current of the electrode closest to said vessel and tothe electrode most distant from said vessel.
 7. The device according toclaim 6, wherein said power supply means includes two generatorsseparately connected to said control means and said electrodes forsupplying electric current to said electrodes.
 8. The device accordingto claim 6, further comprising:a transformer connected to a generator onsaid vessel; said transformer forming a part of said power supply unit;at least a first and a second controlled current rectifier, each ofwhich has two output terminals; a first output terminal of said firstcurrent rectifier being connected to a first electrode arranged closestto said vessel; a second output terminal of said first current rectifierbeing connected to a first output terminal of said second currentrectifier; said first output terminal of said second current rectifierbeing connected to a second electrode arranged behind said firstelectrode; a second output terminal of said second current rectifierbeing connected to a third electrode arranged behind said secondelectrode; and said current rectifiers being separately operativelyconnected to said control means.
 9. The device according to claim 6,further comprising:at least two DC current generators; each of saidgenerators having two output terminals; a first output terminal of saidfirst DC current generator being connected to a first electrode arrangedclosest to said vessel; a second output terminal of said first DCcurrent generator being connected to a first output terminal of saidsecond DC current generator; said first output terminal of said secondDC current generator being connected to a second electrode arrangedbehind said first electrode; a second output terminal of said second DCcurrent generator being connected to a third electrode arranged behindsaid second electrode; and said DC current generators separately beingoperatively connected to said control means.
 10. The device according toclaims 6, 7, 8, or 9, further comprising:said control means including acentral unit; a memory unit operatively connected to said central unit;driver means operatively connected to said central unit; and said drivermeans being connected to said power supply means.